1. Field of the Invention
The present invention relates to variable frequency AC motors. More particularly, the present invention relates to a method and apparatus to compensate for voltage deviations at motor terminals due to switching time delays in pulse width modulated invertors.
2. Description of the Art
One type of commonly designed induction motor is a three phase motor having three Y-connected stator windings. In this type of motor, each stator winding is connected to an AC voltage source by a separate supply line, the source generating currents therein. Often, an adjustable speed drive (ASD) will be positioned between the voltage source and the motor to control motor speed.
Many ASD configurations include a pulse width modulated (PWM) inverter consisting of a plurality of switching devices. By firing the switching devices in a regulated sequence the PWM inverter can be used to control both the amplitude and frequency of voltage that eventually reach the stator windings. Referring to FIG. 2, an exemplary sequence of high frequency terminal voltage pulses 60 that an inverter might provide to a motor terminal can be observed along with an exemplary low frequency alternating fundamental voltage 62 and related alternating current 69. By varying the widths of the positive portions 63 of each high frequency pulse relative to the widths of the negative portions 64 over a series of high frequency voltage pulses 60, a changing average voltage which alternates sinusoidally can be generated. The changing average voltage defines the low frequency alternating voltage 62 that drives the motor. The low frequency alternating voltage 62 in turn produces a low frequency alternating current 69 that lags the voltage by a phase angle .phi..
Many conventional motor applications require highly accurate alternating voltages over a wide speed range. Enhanced control of the alternating voltage can be achieved using a PWM inverter by increasing the frequency of the high frequency voltage pulses 60.
While advanced digital electronic signal generators can produce the desired high frequency signals to turn inverter components ON and OFF, the inverter components cannot turn ON and OFF instantaneously. Referring to FIG. 1, an exemplary inverter 10 has six switches 12-17. The switches 12-17 are arranged in series pairs, each pair forming one of three inverter legs 39, 40, and 41. Referring to leg 39, by turning the switches 12, 13 ON and OFF in a repetitive sequence, leg 39 receives DC voltage 18 and provides the high frequency pulses 60 of FIG. 2 to a motor terminal 31. Ideally, when one switch 12 turns on, the series switch 13 turns OFF, and visa versa.
In reality, however, each switch 12, 13 has turn-on and turn-off times that vary depending on the technology used for their construction. Thus, while signals to turn the upper switch 12 ON and the lower switch 13 OFF might be given at the same instant, the lower switch 13 may go ON faster than the upper switch 12 goes OFF thus providing an instantaneous DC short between a high DC rail 48 and a low DC rail 49. Such a DC short can cause irreparable damage to both the inverter and motor components.
To ensure that the series switches of an inverter are never simultaneously on, a delay module 11 is used to introduce a turn-on delay between the times when one switch turns off and the other switch turns on. Although these turn-on delays are very short, they tend to cause deviations from the precisely designed PWM signal. While each individual deviation does not appreciably affect the fundamental alternating voltage, accumulated deviations do distort the alternating voltage. This is particularly true in applications where the frequency of the high frequency pulses is increased because each additional pulse creates an additional deviation. Accumulated deviations produce torque pulsations, reduce fundamental output voltage, and distorted stator winding currents, all of which are undesirable.
To compensate for the deviations in PWM signals the industry has tried various methods of adding or subtracting correction waveforms to the PWM signals. While the proper phase for the correction waveform can be easily ascertained, it is difficult to determine the exact voltage gained or lost due to the deviations in the fundamental voltage. Thus, the amplitude of the correction waveform cannot be easily ascertained.
In order to find the proper amplitude for the correction waveform and a stable operating point, some sort of trial and error, or look-up table protocol, must be followed to determine the amount of gain or loss. In addition, once a controlled low frequency alternating fundamental voltage is achieved, if the motor load changes, the alternating fundamental voltage becomes uncontrolled and a new amount of gain or loss must be ascertained in order to regain control of the alternating fundamental voltage. These solutions are costly as they require dedicated hardware and computing time from a motor controller that could be used to monitor other motor parameters and operate other motor functions.
Thus, it would be advantageous to have a method and/or apparatus which could compensate for turn on delay in a PWM inverter that is not operating point specific or load dependent and that does not require a look-up table or trial and error protocol to determine how to alter PWM signals.